This invention generally relates to the field of communication systems and, more particularly, to digital communication systems that supports multiple modulation schemes.
Digital communication systems use a variety of linear and non-linear modulation schemes to communicate voice or data information. These modulation schemes include, Gaussian Minimum Shift Keying (GMSK), Quadrature Phase Shift Keying (QPSK), Quadrature Amplitude Modulation (QAM), etc. GMSK modulation scheme is a non-linear low level modulation (LLM) scheme with a symbol rate that supports a specified user bit rate. In order to increase user bit rate, high-level modulation (HLM) schemes can be used. Linear modulation schemes, such as QAM scheme, may have different level of modulation. For example, 16QAM scheme is used to represent the sixteen variation of 4 bits of data. On the other hand, a QPSK modulation scheme is used to represent the four variations of 2 bits of data. Although 16QAM scheme provides a higher bit rate than QPSK, both of these modulation schemes could have the same symbol rate. Application of modulation schemes, however, differ in many aspects, for example symbol rate and/or burst format, which complicates their support in systems that use multiple modulation schemes.
In wireless digital communication systems, standardized air interfaces specify most of system parameters, including modulation type, burst format, communication protocol, symbol rate, etc. For example, European Telecommunication Standard Institute (ETSI) has specified a Global System for Mobile Communications (GSM) standard that uses time division multiple access (TDMA) to communicate control, voice and data information over radio frequency (RF) physical channels or links using GMSK modulation scheme at a symbol rate of 271 ksps. In the U.S., Telecommunication Industry Association (TIA) has published a number of Interim Standards, such as IS-54 and IS-136, that define various versions of digital advanced mobile phone service (D-AMPS), a TDMA system that uses a Differential QPSK (DQPSK) modulation scheme for communicating data over RF links.
TDMA systems subdivide the available frequency band into one or several RF channels. The RF channels are divided into a number of physical channels corresponding to time slots in TDMA frames. Logical channels are formed from one or more physical channels, where modulation and channel coding schemes are specified. In these systems, the mobile stations communicate with a plurality of scattered base stations by transmitting and receiving bursts of digital information over uplink and downlink RF channels.
The growing number of mobile stations in use today has generated the need for more voice and data channels within cellular telecommunication systems. As a result, base stations have become more closely spaced, with an increase in interference between mobile stations 12 operating on the same frequency in neighboring or closely spaced cell. Although digital techniques gain more useful channels from a given frequency spectrum, there still remains a need to reduce interference, or more specifically to increase the ratio of the carrier signal strength to interference, (i.e., carrier-to-interference (C/I)) ratio. RF links that can handle lower C/I ratios are considered to be more robust than those that only can handle higher C/I ratios.
In order to provide various communication services, a corresponding minimum user bit rate is required. For example, for voice and/or data services, user bit rate corresponds to voice quality and/or data throughput, with a higher user bit rate producing better voice quality and/or higher data throughput. The total user bit rate is determined by a selected combination of techniques for speech coding, channel coding, modulation scheme, and for a TDMA system, the number of assignable time slots per call.
Depending on the modulation scheme used, link quality deteriorates more rapidly as C/I levels decrease. Higher level modulation schemes are more susceptible to low levels of C/I ratio than lower level modulation schemes. If a HLM scheme is used, the data throughput or grade of service drops very rapidly with a drop in link quality. On the other hand, if a LLM scheme is used, data throughput or grade of service does not drop as rapidly under the same interference conditions. Therefore, link adaptation methods, which provide the ability to change modulation and/or coding based on the channel conditions, are used to balance the user bit rate against link quality. Generally, these methods dynamically adapt a system's combination of speech coding, channel coding, modulation, and number of assignable time slots to achieve optimum performance over a broad range of C/I conditions.
One evolutionary path for the next generation of cellular systems is to use high-level modulation (HLM), e.g., 16QAM modulation scheme, to provide increased user bit rates compared to the existing standards. These cellular systems include enhanced GSM systems, enhanced D-AMPS systems, International Mobile Telecommunication 2000 (IMT-2000), etc. A high level linear modulation, such as 16QAM modulation scheme, has the potential to be more spectrum efficient than, for example, GMSK, which is a low-level modulation (LLM) scheme. Furthermore, the use of 16QAM modulation scheme in conjunction with a higher symbol rate significantly increase the user bit rate compared to the GMSK modulation scheme. In this way, the maximum user bit rate offered by an HLM scheme, such as 16QAM modulation scheme, may be more than doubled. Because higher level modulation schemes require a higher minimum C/I ratio for acceptable performance, their availability in the system becomes limited to certain coverage areas of the system or certain parts of the cells, where more robust links can be maintained. However, a system can be planned to provide full coverage for HLM scheme. The modulation schemes provided in a cell may be a mixture of non-linear and linear modulation, with different symbol rates.
Generally, two types of logical channels are defined by air interface standards: control channels (CCH) and traffic channels (TCH). CCHs are used for control signalling such as registration, authentication, call set-up, and the like. TCHs, which are single user channels, are used to handle voice or data communication. For TCHs, some of the standards define various user bit rates.
In GSM systems, control signalling is carried out using different types of CCHs, including dedicated control channels (DCCHs), Broadcast Channels (BCHs), and Common Control Channels (CCCHs). BCHs include Frequency Correction channel (FCCH), Synchronization Channel (SCH), and Broadcast Control Channel (BCCH). The CCCHs include Paging channel (PCH), Access Grant Channel (AGCH) and Random Access Channel (RACCH). DCCHs include Stand-alone Dedicated Control Channel (SDCCH), Fast Associated Control Channel (FACCH), and Slow Associated Control Channels (SACCH).
FCCH indicates a BCCH carrier signal and enables a mobile station to synchronize to its frequency. SCH is used to signal TDMA frame structure in a cell and a Base Station Identity Code (BSIC) that indicates whether a base station belongs to a GSM system or not. BCCHs is transmitted a during predefined time slot (e.g., time slot 0 in single carrier base stations) of a downlink RF channel, to provide general information to the mobile stations. SDCCH, which may be transmitted at a time slot adjacent to BCCH, is used for registration, location updating, authentication and call set-up. PCH is a downlink only channel, which is used for informing the mobile station 12 of a network's signaling requirement, for example when the mobile unit is called. AGCH is a downlink only channel used for replies to access requests for assigning a dedicated control channel for a subsequent signaling. RACH is used by a mobile station to request a channel, when it is paged, or when it wants to initiate a call.
The associated control channels, FACCH and SAACH are always associated with traffic channels. Applicable standards specify a number of bits for FACCH and SACCH, which are communicated according to a pre-defined format. SACCH is used for communicating control and supervisory signals associated with traffic channels, including the transmission of parameters corresponding to a measure of bit error rate (BER) or a measure of received signal strength (RSS) at mobile stations 12. FACCH steals bursts allocated for traffic channels for control requirements, such as hand-over.
Fast signaling procedures are needed to quickly provide signalling information to the receiver. For example, in GSM systems, stealing flags, which are time-multiplexed at predefined positions within a burst, are used to distinguish between a FACCH burst and a TCH burst. By reading the stealing flags, the receiver determines the type of logical channels.
In systems that support multiple modulation schemes, demodulation of information communicated over control channels and traffic channels creates many complications. By introduction of link adaptation algorithms, adaptation of coding and/or modulation scheme becomes more frequent. The frequent link adaptations result in an increased signalling effort, causing degradation in communication quality. Furthermore, the control information communicated over FACCHs and voice or data communicated over TCHs must be demodulated without significant overhead in order to improve communication quality.
Therefore, there exists a need for an efficient and simple method for demodulating information in systems that support multiple modulation schemes.